This invention relates to an improved automatic span correcting device for weighers. The inventive device is especially useful when incorporated in a so-called combination weighing machine.
In a weigher having a digital indicator, the digital value on the indicator does not always coincide with the digital value which corresponds to the analog weight of product put on the weighing cradle of the weigher. For example, assume a digital value of "1000" on the indicator is to correspond to an analog weight of "100 grams". If the analog output of the weigher is converted by an analog to digital converter into a digital value of "1010" when the weigher is loaded with product of 100 gram weight, the digital value must be corrected to "1000". Such correction is referred to as "span correction" in the field of weighing equipments. This invention relates to a device for automatically effecting such correction.
The opened Japanese patent specification No. 58-95220 discloses a method of effecting such span correction. In this method, a weigher is first zero-corrected in its unloaded state. Then, the weigher is loaded with a reference test weight having a known weight value and the resultant weight signal is divided by the known weight value to obtain a span correction coefficient which is stored in a register. Thereafter, any weight signal output from the weigher is multiplied by this coefficient to obtain a corresponding span-corrected value of the weight.
In this method, however, in order to obtain accurate measurements the reference test weight must be sufficiently high and the test weight must be changed when the weight of the product is changed, in order to obtain accurate measurements. However, it is troublesome and time-consuming to manually input the weight value of each test weight every time and this problem is especially severe in a combination weighing machine having a number of weighers.